This past weekend I camped at Ft Mountain State Park in northwestern Georgia. One of my goals was to test NVIS and see if it would work during my stay. I collected data using FT8 from Friday afternoon when I got set up until about 2130 hrs eastern when the 40-meter band started to fade.
Conditions. I set up in the campground in Ft Mountain State Park. My elevation was approx. 2,400 feet above sea level located in a depression (-200 Ft.). The antenna was a Chameleon EmComm II ver.1 in an inverted vee configuration. The ends were 6-6.5 feet off of the ground and the apex was 10-12 feet. There was a counterpoise run beneath the antenna. The radio was an Elecraft K3(s) running FT8 at 25-35 watts.
During my stay I made 121 contacts on 40 meters. 7-8 MHz is considered the upper limit for NVIS operation. NVIS is also not guaranteed as it relies on proper atmospheric conditions to reflect the radio waves back downward. When creating the following maps, I only included states where I had 3 or more contacts. In addition, some states had no contacts and that may have been due to propagation, or no one was on at the time. Also, I reported the average signal strength for the state (not individual stations) as well as the percentage of the contacts from that state based upon 121 contacts. If you add up all of the percentages on the following maps, they will not equal 100%.
The first map shows all of the states with 3 or more contacts.
Generally, a good signal for FT8 is any signal above -10dB. A closer view shows that except for Texas the stronger signals are within 300-400 miles of the transmitting station. The circle in the map below has a radius of about 350 miles. As you can see there is a rather sharp drop off in signal strength beyond about 350 miles.
While this isn’t a proper detailed experiment, some generalizations can be made. The transmitting station was located in a depression (-200 Ft), there is a good probability that NVIS was used. The signals close in were stronger, meaning that the signals were within the skip zone and that signal strength beyond the skip zone (beyond about 350 miles) noticeably dropped off indicating NVIS operation. There were more stations contacted within the circle. As the transmitting station turned away from the sun the ability to copy close in stations diminished. The white elephant in the room is Texas. In my experience, there always seems to be a radio pathway between Texas and Georgia. Not sure why that is but in all my activations and at home there always seems to be at least one Texas station in the mix.
As amateur radio operators, we often think in terms of how far we can reach, how many miles per watt, to reach that atoll on the other side of the world. What is equally important and in some cases such as EmComm, is how close can we get. Sometimes the most important link is just on the other side of the mountain. Knowing how to utilize NVIS in your comms plans will help bridge that gap. You may be saving a life instead of collecting a QSL card. Below are some links to other NVIS posts I created. — Scott
KK4Z 